Ischemia is a condition of blood deprivation that affects biological tissue, typically because of a blockage or constriction of the blood vessels supplying the tissues in question. Myocardial ischemia is a common complication of coronary artery disease due to narrowing of the blood vessels because of the accumulation of plaque. In the absence of proper treatment, ischemic tissue may become infarcted (necrotic). Myocardial infarction is a serious occurrence and may result in death. Proper treatment of ischemic areas continues to be a challenge for medical science.
In recent years the concept of revascularizing the myocardium has become the subject of increasing study. If the tissue has remained viable despite the previous deprivation of blood, revascularization, or the restoration of blood flow, to dormant or hibernating tissue can restore the muscle's normal function. The technique of revascularizing the myocardium by creating passages into the tissue through which blood may flow has become known as Transmyocardial Revascularization (TMR). Creating channels part of the way through the myocardium is believed to permit blood from the ventricle to reach sinusoids within the muscle. Early researchers reported promising results by piercing the myocardium to create multiple channels for blood flow. Sen, P. K. et al., "Transmyocardial Acupuncture--A New Approach to Myocardial Revascularization", Journal of Thoracic and Cardiovascular Surgery, Vol. 50, No. 2, August 1965, pp. 181-189. An alternative method involves the use of a laser to form the channels with heat energy. Mirhoseini, M. et al., "Revascularization of the Heart by Laser", Journal of Microsurgery, Vol. 2, No. 4, June 1981, pp. 253-260. The use of a catheter-based apparatus to create laser-made channels for TMR is disclosed in U.S. Pat. No. 5,769,843 (Abela). Abela '843 also discloses the use of a magnetic navigation system to guide the catheter to the desired position within the heart. U.S. Pat. Nos. 5,380,316 and 5,389,096 (Aita) disclose another approach to a catheter-based system for TMR.
Although there has been some published recognition of the desirability of performing TMR in a non-laser catheterization procedure, there does not appear to be evidence that such procedures have been put into practice. U.S. Pat. No. 5,429,144 (Wilk) discloses inserting an expandable implant within a preformed channel created within the myocardium for the purposes of creating blood flow into the tissue from the left ventricle. Performing TMR by placing stents in the myocardium is also disclosed in U.S. Pat. No. 5,810,836 (Hussein et al.). The Hussein patent discloses several stent embodiments that are delivered through the epicardium of the heart, into the myocardium and positioned to be open to the left ventricle.
Recently, researchers have examined the possibility of treating diseased myocardial tissue with therapeutic substances or cell therapy to revive the tissue. For example stem cells, as well as cell components, such as DNA and proteins, are considered to hold potential as a promising treatment for diseased tissue regions. It has been reported that stem cells may be capable of transforming into a highly specialized cells of a given organ in which they are placed. J. Hescheler et al., Embryotic Stem Cells: A Model To Study Structural And Functional Properties In Cardiomyogenesis, Cardiovascular Research 36 (1997) 149-162. Addition of such cells to the tissue of an organ may serve to initiate growth of the tissue of that organ. Also, encouraging blood vessel growth to provide new supplies of oxygenated blood to a region of tissue has been reported as a potential remedy for a variety of tissue and muscular ailments, particularly ischemia. Primarily, study has focused on perfecting angiogenic factors such as human growth factors produced from genetic engineering techniques. It has been reported that injection of such a growth factor into myocardial tissue initiates angiogenesis at that site, which is exhibited by a new dense capillary network within the tissue. Schumacher et al., "Induction of Neo-Angiogenesis in Ischemic Myocardium by Human Growth Factors", Circulation, 1998; 97:645-650.
Because accurate diagnosis and identification of ischemic areas is essential to proper treatment, there is a need for medical techniques that can pinpoint ischemic regions of the myocardium. Previous methods of ischemic tissue identification have relied on various techniques mostly relying on visual identification of the ischemic region. In identifying ischemic areas of myocardial tissue of the heart, movement of the heart muscle is observed for abnormal motion. Such visual methods can require extensive x-ray imaging. PET scanning, Thalium 201, and magnetic resonance imaging are examples of current visual methods of identifying ischemic regions. Recently, Gamma Camera Imaging has been reported as a potentially useful tool in identifying ischemic myocardial tissue. Okada et al., "Tc-HL91 "Hot Spot" Detection of Ischemic Myocardium In Vivo by Gamma Camera Imaging" Circulation, 1998;97:2557-2566.
It would be desirable to provide an accurate yet simple and cost effective method of ischemia detection and identification that could be easily combined with a procedure for treating the identified ischemic region. It is an object of the present invention to provide such a system.